Copper Electroplating Solution and Copper Electroplating Process

ABSTRACT

The present disclosure relates to a copper electroplating solution and a copper electroplating process. The solution includes following components: 20 to 240 g/L of copper sulfate pentahydrate, 20 to 300 g/L of sulfuric acid, 25 to 120 mg/L of chlorine ion, 0.1 to 20 mg/L of a brightener, 1 to 2000 mg/L of an inhibitor, and the balance is deionized water; The brightener is selected from two of the group consisting of alkyl sulfonic acid thiols and derivatives thereof; the inhibitor is selected from one or more compounds of non-ionic surfactants. The solution of this disclosure can greatly improve the current density of plating and the throwing power (TP) of electroplating of the through hole of flexible boards, wherein the TP value can reach more than 200%, and the electroplating deposited copper layer in the hole is flat and the quality thereof meets the requirements of the flexible board.

FIELD OF THE DISCLOSURE

The present disclosure relates to the field of printed circuit board manufacturing technology, and more particularly to a copper electroplating solution and a copper electroplating process.

BACKGROUND OF THE DISCLOSURE

A Flexible Printed Circuit Board is also known as “FPC Flexible Board”, which is a printed circuit board manufactured by flexible insulating base materials. The flexible board with the characteristics of small size and light weight, can greatly reduce the size of the device to meet the needs of development of high density, miniaturization, lightweight, thin type and high reliability for electronic products. In addition, the flexible board has high flexibility, for example, it can be bended, winded, twisted, folded, and changed in shape randomly according to the spatial layout requirements, and it can also be moved and expand freely in any three-dimensional space to realized the integration of component assembly and wire connection. Meanwhile, the FPC flexible board also has the advantages of good heat dissipation, weldability and easy assembly. The flexible circuit board is widely used in the field of aerospace, military, mobile communication, portable computer, computer peripherals, PDA, digital camera and the like due to its unique characteristics. Compared with the rigid circuit board, the manufacturing process and the process equipment of the flexible circuit board are basically the same as those of the rigid board. However, the flexible nature of the flexible board leads to a more complex and more difficult manufacturing process thereof.

At present, there are mainly three ways for electrical interconnection between boards: blind via, through hole and buried hole. The electrical interconnection of the PCB realized by the three methods is mainly achieved through copper electroplating technology. The most used copper electroplating solution currently is sulfate type electroplating solution, since the plating layer obtained is uniform, fine and soft, and the plating solution has simple composition, good dispersing ability, good throwing power, high current efficiency, high deposition rate, and the sewage treatment is simple. In addition to copper sulfate, sulfuric acid, chlorine ion and other inorganic components, some different types of organic additives are also needed to be added into the copper sulfate electroplating solution, to adjust the current distribution during the electroplating process and to improve the even-plating capacity of the plating solution. The commonly used organic additives are brighteners, inhibitors and leveling agents. The brightener is usually a small molecular sulfur-containing organic compound, which is favorable for the formation of nucleation during electroplating process, so that the crystal nuclei are densely distributed and the copper plating layer becomes smooth and reflective. The typical functionalized functional groups thereof are disulfide bond (—S—S—), sulfonic acid group (—SO₃—) and mercapto group (—SH). The commonly used brighteners are bis-(sodium sulfopropyl)-disulfide (SPS), and sodium 3-mercaptopropane sulphonate (MPS). Most of the inhibitors are macromolecule oxygenated compounds, which are adsorbed on the surface of the cathode under the synergistic effects of chloride ions to restrain the deposition of metallic copper on the surface. Meanwhile, the inhibitor may act as a wetting agent, reducing the surface tension of interface (reducing the contact angle) and allowing the plating solution to enter a hole more easily to increase mass transfer effect. The leveling agents are usually nitrogen-containing organic compounds with very strong positive electricity, which are readily adsorbed in high current density areas (raised areas or corners), and compete with copper ions, to slow the electroplating speed herein without influencing the electroplating in low current density areas (recessed areas), and make the original undulating surface more flat.

There are mainly two indicators measuring the electroplating performances of the through hole of a flexible board in the industry: TP value and thermal stress test, wherein TP value is the main indicator. TP is throwing power, representing the ability of an electroplating solution to deposit metallic plating layers in deep recesses of parts. The value of the TP is the percentage of the thickness of the copper layer in the hole and the thickness of the copper layer on the board. The higher the TP value, the stronger the ability to deposit metals in the deep recesses of parts, the higher the reliability of electrical interconnection, thus ensuring the further processing of the flexible board. If the TP value is too low, the metals may not be deposited in the deep recesses of the parts, resulting in an open-circuit phenomenon, or the copper layer in the hole will be thinner and thinner during the subsequent processes of the PCB, eventually leading to an open-circuit phenomenon. The thermal stress test is to test the combination of the new copper layer and the board at high temperature to ensure the reliability thereof during high temperature welding.

It is the primary goal in the field of flexible board electroplating to improve the TP value of the through hole electroplating of the flexible board. Research work shows that the leveling agent in a plating solution will greatly reduce the electroplating effect of the through hole of a flexible board, so the leveling agent can not be added during the electroplating of the through hole of the flexible board. The present disclosure believes that the reason thereof is the significant difference between the flexible board and the rigid board: the flexible board is thinner. The thickness of the through hole of the whole flexible board is equivalent to the thickness of the hole openings on both sides of the through hole of the rigid board, that is, the hole of the thin flexible board is equivalent to the hole opening of the rigid board, and the main action site of the leveling agent is the hole opening. On the other hand, the thinner the board, the smaller the thickness-diameter ratio, the easier exchange of solution inside and outside the hole, which is beneficial for electroplating copper to the through hole of a thick rigid board. However, with respect to the flexible board with very small thickness, easier solution exchange also means that the distribution inside and outside the hole of the organic additive will also change with the solution exchange and the leveling agent which mainly works on the board will be more easily distributed into the hole and inhibits the copper deposition in the hole, whereas the accelerator originally adsorbed in the hole will be adsorbed more on the board to accelerate the copper deposition on the board since the adsorption sites are occupied by the leveling agent, resulting in a decrease of the TP value. Therefore, the three-component formulation commonly used in rigid PCBs is not suitable for the copper electroplating of the through hole of a flexible board, and it is imperative to develop a copper plating formulation specifically for flexible boards.

SUMMARY OF THE DISCLOSURE

Based on the above, the purpose of the present disclosure is to provide a copper electroplating solution suitable for flexible boards.

The specific technical solution is described below.

A copper electroplating solution, comprising the following components:

copper sulfate pentahydrate: 20 to 240 g/L

sulfuric acid: 20 to 300 g/L

chlorine ion: 25 to 120 mg/L

brightener: 0.1 to 20 mg/L

inhibitor: 1 to 2000 mg/L

deionized water: balance;

The brightener is selected from two compounds of the group consisting of alkyl sulfonic acid thiols or derivatives thereof; the inhibitor is selected from one or more of non-ionic surfactants.

In some embodiments, the brightener is bis-(sodium sulfopropyl)-disulfide and sodium N, N-dimethyl dithiocarboxamide propanesulfonate.

In some embodiments, the added amount of bis-(sodium sulfopropyl)-disulfide is 0.1 to 10 mg/L; the added amount of sodium N, N-dimethyl dithiocarboxamide propanesulfonate is 0.1 to 10 mg/L.

In some embodiments, the sum of the added amount of bis-(sodium sulfopropyl)-disulfide and the added amount of sodium N, N-dimethyl dithiocarboxamide propanesulfonate is greater than 0.5 mg/L, and the absolute value of the difference between the added amount of bis-(sodium sulfopropyl)-disulfide and the added amount of sodium N, N-dimethyl dithiocarboxamide propanesulfonate is more than 0.5 mg/L. Preferably, the sum of the added amount of bis-(sodium sulfopropyl)-disulfide and the added amount of sodium N, N-dimethyl dithiocarboxamide propanesulfonate is greater than 1 mg/L and less than 6 mg/L.

In some embodiments, the inhibitor is selected from one or more compounds of the group consisting of polyalkylene glycol compounds, polyvinyl alcohol, carboxymethyl cellulose, polyethylene glycol, polyethylene glycol stearate, alkoxy naphthol, oleic acid polyglycol ester, poly (ethylene glycol-propylene glycol) random copolymer, poly (polyethylene glycol-polypropylene glycol-polyethylene glycol) block copolymer, poly (polypropylene glycol-polyethylene glycol-polypropylene glycol) block copolymer, and the added amount of the inhibitor is in the range of 1 to 2000 mg/L, preferably 500 to 1000 mg/L.

Another purpose of the present disclosure is to provide a copper electroplating process for a flexible printed circuit board.

The specific technical solution is described below.

A copper electroplating process for a flexible printed circuit board, including pretreatment procedure, first copper electroplating procedure, rinsing procedure, second copper electroplating procedure and post treatment procedure;

The copper electroplating solution according to any one of claims 1-5 is used in the first copper electroplating procedure;

a second copper electroplating solution is used in the second copper electroplating procedure, and the second copper electroplating solution includes the following components:

copper sulfate pentahydrate: 20 to 240 g/L

sulfuric acid: 20 to 300 g/L

chlorine ion: 25 to 120 mg/L

second brightener: 0.1 to 20 mg/L

second inhibitor: 1 to 2000 mg/L

leveling agent: 5 to 40 mg/L

deionized water: balance;

The second brightener is selected from one of the group consisting of bis-(sodium sulfopropyl)-disulfide, sodium mercaptopropanesulfonate, 2-mercaptobenzimidazole and ethylene thiourea; The second inhibitor is selected from one or more of the group consisting of polyalkylene glycol compounds, polyvinyl alcohol, carboxymethyl cellulose, polyethylene glycol, polyethylene glycol stearate, alkoxy naphthol, oleic acid polyglycol ester, poly (ethylene glycol-propylene glycol) random copolymer, poly (polyethylene glycol-polypropylene glycol-polyethylene glycol) block copolymer, poly (polypropylene glycol-polyethylene glycol-polypropylene glycol) block copolymer; The leveling agent is selected from one or more of the group consisting of polyethylenimine or a derivative thereof, caprolactam or a derivative thereof, polyvinyl pyrrole or a derivative thereof, diethylenetriamine or a derivative thereof, hexamethylenetetramine or derivatives thereof, dimethyl phenylpyrazolone onium salt or derivatives thereof, rosaniline or derivatives thereof, sulfur-containing amino acids or derivatives thereof, phenazine onium salt or derivatives thereof.

In some embodiments, the process parameters of the first copper electroplating procedure are: 1 to 5 A/dm² of current density, 15 to 32° C. of electroplating temperature, and 20 to 120 min of electroplating time; the process parameters of the second copper electroplating procedure are: 1 to 5 A/dm² of current density, 15 to 32° C. of electroplating temperature, and 1 to 20 min of electroplating time.

Another purpose of the present disclosure is to provide a flexible printed circuit board.

A flexible printed circuit board is prepared by the copper electroplating process described above.

In some embodiments, the size of the micro through hole in the flexible printed circuit board is: 20 to 300 μm in diameter and 40 to 300 μm in the thickness of the board.

The principle and advantages of the present disclosure are as follows:

The additive in the copper electroplating solution of the present disclosure contains three components, wherein two components exert accelerating effect and the other component exerts inhibitory effect. In the two components exerting accelerating effect (bis-(sodium sulfopropyl)-disulfide SPS and sodium N, N-dimethyl dithiocarboxamide propanesulfonate DPS), SPS acts as a main brightener to increase copper deposition rate in the hole and to improve the TP of electroplating of the flexible board, while DPS acts as an auxiliary brightener to accelerate the deposition of copper and to improve the quality of the plating layer as well. DPS has nitrogen, sulfur-containing structure, and in the acidic plating conditions, the nitrogen-containing structure thereof will play a role as an inhibitor and a leveling agent as well to some degree. In addition, compared with the copper plating layer deposited by SPS, the copper plating layer deposited by DPS shows no significant difference in the impurity level and the crystallinity. However, the DPS-plated copper crystal grains are more refined and the deposited layer is more dense, so as to reduce the roughness of the copper layer and further to reduce the resistivity of the copper layer.

In addition, there is no addition of leveling agent in the copper electroplating solution of the present disclosure. Compared with the plating solution with leveling agent, the plating solution without leveling agent will greatly enhance the TP of electroplating of the flexible board, since the plating solution without leveling agent will eliminate the inhibition of copper deposition in holes caused by the leveling agent. Meanwhile, the plating solution without leveling agent will improve the quality of plating layer to a certain extent. Research shows that the leveling agent will continue to be consumed during the electroplating process and the resulting impurities will enter the copper plating layer and increase the stress (i.e. brittleness) of the plating layer. Meanwhile the pollution of the copper plating layer caused by the incorporation of the leveling agent in the deposited copper will cause an increase in the interconnection resistance. Therefore, the plating solution of the present disclosure will solve the quality problems caused by the incorporation of the leveling agent to a certain extent.

The copper electroplating solution of this disclosure can bring the TP value to more than 200%, and the electroplating deposited copper layer in the hole is flat and the quality of the copper plating layer meets the requirements of the flexible board.

Another purpose of the present disclosure is to provide a method of using the plating solution, that is, a copper electroplating process for manufacturing a printed circuit board using the plating solution. A brighter and flatter plating layer can be obtained by this method.

The purpose of this disclosure is achieved by the following technical solution:

Step 1. If necessary, fixing the flexible board with a specific specification on the frame for fixing the flexible board to ensure a good conductivity between the flexible board and the conductive frame;

Step 2. If necessary, degreasing and rinsing the board after through hole conductive treatment to prevent the degreasing agent from remaining on the board and being brought into the subsequent steps;

Step 3. If necessary, presoaking the board after process in step 2;

Step 4. Exerting electroplating (first copper electroplating procedure) on the board after process in step 3 in the copper electroplating solution provided by the present disclosure, and rinsing the board so as to prevent the electroplating solution from remaining on the board and being brought into the subsequent steps;

Step 5. Exerting electroplating (second copper electroplating process) on the board after process in step 4 in the copper electroplating solution (second copper electroplating solution) containing a three-component organic additive (i.e. containing a leveling agent, a brightener, and an inhibitor)

Step 6. Rinsing and drying the plated board, then can enter the next process.

The degreasing agents used in the copper electroplating processes of the printed circuit board above are all commercial products; the presoak involved is a sulfuric acid with the concentration of 10 to 100 g/L.

In the method of using the copper electroplating solution of the disclosure, the current density of electroplating copper is in the range of 1 to 5 A/dm², the electroplating temperature is in the range of 15 to 32° C., and the electroplating time is in the range of 20 to 120 mins in step 4;

In the method of using the copper electroplating solution of the disclosure, in step 5, the copper electroplating solution containing a three-component organic additive may be a commercial product or may be prepared by adding a leveling agent to the plating solution of the present disclosure, and the added amount of the leveling agent is in the range of 5 to 40 mg/L, and the leveling agent involved may be any one or more of the commercial products. The current density is in the range of 1 to 5 A/dm², the electroplating temperature is in the range of 15 to 32° C., and the electroplating time is in the range of 1 to 20 min.

The purpose of the first copper electroplating procedure is to improve the TP of electroplating of a flexible board by using the plating solution of the present disclosure and to ensure the reliability of the electrical interconnection.

The purpose of the second copper electroplating procedure is to improve the problems of board that may be generated in the first copper electroplating procedure and to ensure the surface is bright and flat after the whole electroplating process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow chart of a copper electroplating process using a plating solution provided by the present disclosure;

FIG. 2 shows cross-sectional metallographic micrographs at 500× magnification of flexible boards after electroplating using commercial plating solutions for flexible boards: wherein Figure (a) shows cross-sectional metallographic micrographs at 500× magnification of a double-layer flexible board after electroplating using a Rohm and Haas ST920 plating solution in example 1; Figure (b) shows cross-sectional metallographic micrographs at 500× magnification of a double-layer flexible board after electroplating using a MacDermid VP100 plating solution for flexible boards in example 2; Figure (c) shows cross-sectional metallographic micrographs at 500× magnification of a multi-layer flexible board after electroplating using a MacDermid VP100 plating solution for flexible boards in example 3;

FIG. 3 shows cross-sectional metallographic micrographs at 500× magnification of flexible boards after electroplating using copper electroplating solutions of the present disclosure: wherein Figure (a) shows cross-sectional metallographic micrographs at 500× magnification of a double-layer flexible board after electroplating using a copper electroplating solution of the present disclosure in example 4; Figure (b) shows cross-sectional metallographic micrographs at 500× magnification of a multi-layer flexible board after electroplating using a copper electroplating solution of the present disclosure in example 8;

FIG. 4 shows cross-sectional metallographic micrographs at 500× magnification of a double-layer flexible board after electroplating using a copper electroplating process of the present disclosure in example 9.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosure will be illustrated by the following examples. However, the examples are only for the further elaboration of the present disclosure and are not intended to limit the scope of the present disclosure.

In order to test the advantages of the present disclosure, the through holes of flexible boards are electroplated using two commercial plating solutions for through hole according to the optimum conditions required by the operating instructions. Examples 1-3 are examples of using commercial plating solutions, and examples 4-9 are examples of using plating solutions of the present disclosure.

Example 1

COPPER GLEAM™ ST-920 electroplating solution additive (a typical three-component formulation) of Rohm and Haas Company is used in this example, and the electroplating solution is formulated in the following formulation according to the instruction.

Copper sulfate pentahydrate: 70 g/L

Sulfuric acid: 220 g/L

Chlorine ion: 60 mg/L

Each component of the ST920 electroplating solution additive is added at a concentration according to the instruction:

ST920 brightener: 2.75 mL/L

ST920 inhibitor: 17.5 mL/L

ST920 leveling agent: 3.25 mL/L

ST920 stabilizing agent: 5 mL/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 150 μm in diameter and 75 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 20 mins, and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1 and FIG. 2 (a).

Example 2

A MACUSPEC VP100 electroplating solution additive (the formulation does not contain a leveling agent, but the brightener thereof is different from that of the present disclosure) of MacDermid Company is used in this example. The electroplating solution is formulated in the following formulation according to the instruction.

Copper sulfate pentahydrate: 120 g/L

Sulfuric acid: 200 g/L

Chlorine ion: 70 mg/L

Each component of the VP100 electroplating solution additive is added at a concentration according to the instruction:

VP100 brightener: 1 mL/L

VP100 inhibitor: 10 mL/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 56 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 25 mins, and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1 and FIG. 2 (b).

Example 3

A MACUSPEC VP100 electroplating solution additive of MacDermid Company is used in this example. The electroplating solution is formulated in the following formulation according to the instruction.

Copper sulfate pentahydrate: 120 g/L

Sulfuric acid: 200 g/L

Chlorine ion: 70 mg/L

Each component of the VP100 electroplating solution additive is added at a concentration according to the instruction:

VP100 brightener: 1 mL/L

VP100 inhibitor: 10 mL/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 185 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 30 mins, and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1 and FIG. 2 (c).

Example 4

The following compounds are added to deionized water to prepare an electroplating solution.

Copper sulfate pentahydrate: 130 g/L

Sulfuric acid: 170 g/L

Chlorine ion: 60 mg/L

Main brightener SPS: 1.5 mg/L

Auxiliary brightener DPS: 0.5 mg/L

Plating inhibitor (PEG 6000): 500 mg/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 56 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 25 mins, and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1 and FIG. 3 (a).

Example 5

The following compounds are added to deionized water to prepare an electroplating solution.

Copper sulfate pentahydrate: 130 g/L

Sulfuric acid: 170 g/L

Chlorine ion: 60 mg/L

Main brightener SPS: 2 mg/L

Auxiliary brightener DPS: 1 mg/L

Plating inhibitor (PEG 6000): 500 mg/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 56 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 25 mins, and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1.

Example 6

The following compounds are added to deionized water to prepare an electroplating solution.

Copper sulfate pentahydrate: 130 g/L

Sulfuric acid: 170 g/L

Chlorine ion: 60 mg/L

Main brightener SPS: 1.5 mg/L

Auxiliary brightener DPS: 1.5 mg/L

Plating inhibitor (PEG 6000): 1000 mg/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 56 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 25 mins, and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1.

Example 7

The following compounds are added to deionized water to prepare an electroplating solution.

Copper sulfate pentahydrate: 130 g/L

Sulfuric acid: 170 g/L

Chlorine ion: 60 mg/L

Main brightener SPS: 0.5 mg/L

Auxiliary brightener DPS: 1.5 mg/L

Plating inhibitor (PEG 6000): 500 mg/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 56 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 25 mins, and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1.

Example 8

The following compounds are added to deionized water to prepare an electroplating solution.

Copper sulfate pentahydrate: 130 g/L

Sulfuric acid: 170 g/L

Chlorine ion: 60 mg/L

Main brightener SPS: 3.3 mg/L

Auxiliary brightener DPS: 0.7 mg/L

Electroplating inhibitor (PEG 6000): 667 mg/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution above. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 185 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., the electroplating time is 25 mins, and the air flow rate of air stirring is 1.3 L/min. The test results obtained in this example are shown in Table 1 and FIG. 3 (b).

Example 9

In this example, a copper electroplating process (the process flow chart is shown in FIG. 1) using the plating solution of the present disclosure involved in the present disclosure will be described as an example.

The following compounds are added to deionized water to prepare a copper electroplating solution of the first copper electroplating procedure:

Copper sulfate pentahydrate: 130 g/L

Sulfuric acid: 170 g/L

Chlorine ion: 60 mg/L

Main brightener SPS: 1.5 mg/L

Auxiliary brightener DPS: 0.5 mg/L

Plating inhibitor (PEG 6000): 500 mg/L

Deionized water: balance.

The following compounds are added to deionized water to prepare a second copper electroplating solution of the second copper electroplating procedure:

Copper sulfate pentahydrate: 130 g/L

Sulfuric acid: 170 g/L

Chlorine ion: 60 mg/L

Main brightener SPS: 1.5 mg/L

Auxiliary brightener DPS: 0.5 mg/L

Plating inhibitor (PEG 6000): 500 mg/L

Plating leveling agent (Janus green B): 30 mg/L

Deionized water: balance.

After a series of pretreatments for a flexible board with a specific specification, that is, after degreasing, water rinsing, and presoaking, electroplating is performed in the copper electroplating solution I for 25 mins and then in the copper electroplating solution II for 5 mins. In this example, the flexible board to be plated after black hole is used as a cathode, and the soluble phosphorus-containing copper is used as an anode. The specification of the through hole on the flexible board to be plated is: 200 μm in diameter and 56 μm in board thickness. The current density of the electroplating is 2 A/dm², the electroplating temperature is 25° C., and the air flow rate of air stirring is 1 L/min. The test results obtained in this example are shown in Table 1 and FIG. 4.

TABLE 1 The results of the electroplating performance tests of the through holes of flexible boards obtained from Examples 1-9. TP Thermal Board Hole Test items value stress test state opening Commercial Example 1 156% qualified bright slightly electroplating convex solutions Example 2 165% qualified bright flat Example 3 160% qualified bright flat Plating solution Example 4 270% qualified not bright slightly of the present convex disclosure Example 5 250% qualified not bright slightly convex Example 6 150% qualified partly bright flat Example 7 200% qualified not bright slightly convex Example 8 210% qualified bright slightly convex Example 9 200% qualified bright flat

Comparing Examples 1, 2, 3 with Examples 4, 5, 7, 8, 9, it can be seen that the copper electroplating solutions of the present disclosure can greatly improve the throwing power (TP) of electroplating of the through hole of flexible boards. By electroplating with the plating solutions of the present disclosure, the TP value of a thin double-layer flexible board can reach more than 240%. After electroplated with the plating solutions of the present disclosure, the TP value of a laminated multi-layer flexible board can reach more than 200%, and the electroplating deposited copper layer in the hole is flat and the quality of the copper plating layer meets the requirements of the flexible boards.

In Example 6, it can be seen that when the amount of SPS is equal to that of DPS, the TP value of the electroplating will be only about 150%. Therefore, in the electroplating solutions of the present disclosure, the SPS and the DPS should satisfy the following relation: SPS-DPS|>0.5 mg/L.

Comparing Example 4 with Example 9, in Example 4, a plated through hole having high TP and qualified copper can be obtained, but the appearance is not good enough; In Example 9, electroplating is performed at least for 1 min by using an ordinary three-component copper electroplating solution containing a leveling agent, which can significantly improve the appearance and obtain bright plating surface.

Various technical features of the above embodiments can be combined in any manner. For clarity of description, all possible combinations of various technical features of the above embodiments are not described. However, as long as combinations of these technical features do not contradict with each other, they should be regarded within the scope described in the present specification.

The foregoing examples are merely specific embodiments of the present disclosure, which are described in detail, but they are not intended to limit the protection scope of the present disclosure. It should be noted that any variation or replacement readily figured out by persons skilled in the art within the technical scope disclosed in the present disclosure shall all fall within the protection scope of the present disclosure. Therefore, the scope of the present disclosure is defined by the appended claims. 

1. A copper electroplating solution, comprising the following components: copper sulfate pentahydrate: 20 to 240 g/L sulfuric acid: 20 to 300 g/L chlorine ion: 25 to 120 mg/L brightener: 0.1 to 20 mg/L inhibitor: 1 to 2000 mg/L deionized water: balance; wherein, the brightener is selected from two of the group consisting of alkyl sulfonic acid thiols and derivatives thereof; the inhibitor is selected from one or more of non-ionic surfactants.
 2. The copper electroplating solution according to claim 1, wherein the brightener is bis-(sodium sulfopropyl)-disulfide and sodium N, N-dimethyl dithiocarboxamide propanesulfonate.
 3. The copper electroplating solution according to claim 2, wherein the bis-(sodium sulfopropyl)-disulfide is added at an amount of 0.1 to 10 mg/L; the sodium N, N-dimethyl dithiocarboxamide propanesulfonate is added at an amount of 0.1 to 10 mg/L.
 4. The copper electroplating solution according to claim 3, wherein the sum of the added amount of bis-(sodium sulfopropyl)-disulfide and the added amount of sodium N, N-dimethyl dithiocarboxamide propanesulfonate is greater than 0.5 mg/L, the absolute value of the difference between the added amount of bis-(sodium sulfopropyl)-disulfide and the added amount of sodium N, N-dimethyl dithiocarboxamide propanesulfonate is more than 0.5 mg/L.
 5. The copper electroplating solution according to claim 4, wherein the sum of the added amount of bis-(sodium sulfopropyl)-disulfide and the added amount of sodium N, N-dimethyl dithiocarboxamide propanesulfonate is greater than 1 mg/L and less than 6 mg/L.
 6. The copper electroplating solution according to claim 1, wherein the inhibitor is selected from one or more of the group consisting of polyalkylene glycol compounds, polyvinyl alcohol, carboxymethyl cellulose, polyethylene glycol, polyethylene glycol stearate, alkoxy naphthol, oleic acid polyglycol ester, poly (ethylene glycol-propylene glycol) random copolymer, poly (polyethylene glycol-polypropylene glycol-polyethylene glycol) block copolymer, poly (polypropylene glycol-polyethylene glycol-polypropylene glycol) block copolymer, and the inhibitor is added at an amount in the range of 1 to 2000 mg/L.
 7. A copper electroplating process for flexible printed circuit board, comprising pretreatment procedure, first copper electroplating procedure, rinsing procedure, second copper electroplating procedure and post treatment procedure; wherein the copper electroplating solution according to claim 1 is used in the first copper electroplating procedure; a second copper electroplating solution is used in the second copper electroplating procedure, and the second copper electroplating solution includes the following components: copper sulfate pentahydrate: 20 to 240 g/L sulfuric acid: 20 to 300 g/L chlorine ion: 25 to 120 mg/L second brightener: 0.1 to 20 mg/L second inhibitor: 1 to 2000 mg/L leveling agent: 5 to 40 mg/L deionized water: balance; wherein the second brightener is selected from one of the group consisting of bis-(sodium sulfopropyl)-disulfide, sodium mercaptopropanesulfonate, 2-mercaptobenzimidazole and ethylene thiourea; the second inhibitor is selected from one or more of the group consisting of polyalkylene glycol compounds, polyvinyl alcohol, carboxymethyl cellulose, polyethylene glycol, polyethylene glycol stearate, alkoxy naphthol, oleic acid polyglycol ester, poly (ethylene glycol-propylene glycol) random copolymer, poly (polyethylene glycol-polypropylene glycol-polyethylene glycol) block copolymer, poly (polypropylene glycol-polyethylene glycol-polypropylene glycol) block copolymer; and the leveling agent is selected from one or more of the group consisting of polyethylenimine or derivatives thereof, caprolactam or derivatives thereof, polyvinyl pyrrole or derivatives thereof, diethylenetriamine or derivatives thereof, hexamethylenetetramine or derivatives thereof, dimethyl phenylpyrazolone onium salt or derivatives thereof, rosaniline or derivatives thereof, sulfur-containing amino acids or derivatives thereof, phenazine onium salt or derivatives thereof.
 8. The copper electroplating process for flexible printed circuit board according to claim 7, wherein process parameters of the first copper electroplating procedure are: 1 to 5 A/dm² of current density, 15 to 32° C. of electroplating temperature, and 20 to 120 mins of electroplating time; process parameters of the second copper electroplating procedure are: 1 to 5 A/dm² of current density, 15 to 32° C. of electroplating temperature, and 1 to 20 mins of electroplating time.
 9. A flexible printed circuit board prepared by the copper electroplating process according to claim
 7. 10. The flexible printed circuit board according to claim 9, wherein the flexible printed circuit board is provided with a micro through hole having a size of 20 to 300 μm in diameter and 40 to 300 μm in board thickness. 